Formation Of Several Microstructures In Biomineralization:Mechanism Of Nucleation And Interfacial Growth

Biominerals are unique mineral assemblies generated by biological activities,which usually possess extraordinary morphologies and physical properties.Sophisticated hierarchical structures could be found in various biominerals,which could significantly enhance the mechanical performance of materials.Moreover,some biominerals could be molded by cellular-event-induced organic templates and indicate curved morphologies,or infiltrate into pores of protein fibers to form ultra-thin nanocrystals.Understanding the growth and nucleation mechanisms of biomineralization process will contribute to the development of biomimetic materials and medical technology as well as the synthesis of functional crystal materials.In this thesis,we fabricated several biomineral microstructures via biomimetic mineralization experiments and investigated their formation mechanisms.Key factors that regulate the product morphologies are studied such as intrinsic crystal properties and organic-inorganic interactions.Using in-situ CryoTEM technique,we also investigated the growth and nucleation mechanisms of the amorphous precursor of biomineralization process.These discoveries are benefical for a better understanding of biomineralization process and will shed light to material and medical researches.The detailed discoveries are as follows:1.A hydroxyapatite(HAP)/Chondrotin Sulfate(ChS)composite film with column-like microstructure is fabricated for the first time using hexamethylene-tetramine(HMT)hydrolysis method.Control experiments indicate that film-like product forms under a solution evaporation rate of 2.5 mL/h,while spherulites are generated when the solution is sealed.ChS could reduce the sizes of the spherulites,while F-ion could enhance the orientation of the products.The results indicate that the film is formed via the self-assembly of HAP spherulites at solution-air interface.The growth of the adjacent spherulites are limited by each others and generate the column-like microstructure.2.A pumpkin-shaped vaterite hierarchical structure is synthesized using ammonia carbonate diffusion method.The growth process is in-situ observed.The surface of the structure is composed by lamellar stacked vaterite crystal plates.The entire structure possesses hexagonal symmetry.In-situ observations indicate that the structure is formed via a double-leaf spherulite growth process.When fully crystallized,well-facet and oriented aligned hexagonal vaterite crystal plates appear in the central hexagonal concave of the structure,indicating the hexagonal symmetry of the structure is induced by the hexagonal crystal structure of vaterite.Control experiments indicate that the growth of the structure is regulated by supersaturation:no branching happens when Ca2+ concentration is reduced from 5 mM to 0.1 mM,while high level of branching happens at Ca2+ concentration of 100 mM and generates secondary branching structures.Since the polarized(0001)face of vaterite crystal could generate intrinsic dipole field,we assume that the<0001>directions of the branched vaterite crytal plates might follow the direction of the dipole field.3.The growth and nucleation process of polarized organic molecule induced CaCO3 thin film is in-situ studied for the first time using CryoTEM technique.DNA is used to induce the thin film formation.The results show that amorphous CaCO3(ACC)nanoparticles form in the early growth stage.The nanoparticles could cover on hydrophilic substrate and generate thin film,which later transfrom into CaCO3 crystal plates.Tomography results indicate the surface of the nanoparticles are absorbed by fiber like DNA molecules.Furthermore,there are?2 nm sized sub-units within the nanoparticles.FTIR and EDX show that the early ACC products contain DNA.The DNA ratio decreased after crystallization.In the CaCO3 crystal plates that transformed from ACC thin films,parallel aligned curved patterns could be found.The distance between the patterns is?3 nm.HRTEM results reveal that the patterns are composed of?3 nm sized vaterite crystals.The(1120)planes of these crystals are oriented with the edges of the patterns.4.CaCO3 crystal layers with a thickness of?1 nm was synthesized for the first time using ammonia carbonate diffusion method..A smectic liquid crystal film composed of 6OBA/C6H molecules is used as the growth template.After base treatement,the hydrogen bridges within the film break and generate?1 nm thick gaps.Poly-aspartic acid(pAsp)is used to promote the infiltration of CaCO3.The results show that in the quickly-crystallized region,the CaCO3 could not infiltrate into the film.FTIR of the membrane after 24 h of CaCO3 growth shows peak at 1675 cm-1 corresponding to vibration of the hydrogen bonds,indicating that the bonds are reestablished.After 7 d of growth,the H-bond peak is stronger.Control experiments show that ACC nanolayers are formed at when pAsp concentration is increased from 25 to 100 mg/l,while FTIR shows no H-bond peak.This indicates that the pAsp could inhibit the crystallization of CaCO3 as well as the reestablishment of H-bonds,thus keep the gaps open and promote the infiltration.